Tracking systems based on the magnitude and phase differences of electromagnetic field generated at an external reference point and sensed by a device attached to the tracked object, are known in the patent literature.
U.S. Pat. No. 3,121,228 to Kalmus describes a system for finding the two dimensional location and direction of a vehicle in which a fixed generator and sensor coils on the vehicle are in the same plane, i.e. on the ground. The generator includes coils which produce a rotating magnetic dipole field. The generator and the sensor are each made of a set of two orthogonal coils. The generator's coils are energized with a 90 degrees phase difference by an alternating current source. The orientation of the sensors is obtained based on the phase difference between the signals generated in the sensor coils and the distance from the transmitter is based on the sum of the induced signals.
U.S. Pat. Nos. 3,868,565 and 3,983,474 to Kuipers details a system in which direction is measured by a nutating electromagnetic field in which a pointing vector tracks the direction of the sensor.
U.S. Pat. No. 4,054,881 to Rabb describes a system including three mutually orthogonal radiating antennas, each of which transmits electromagnetic radiation to three mutually orthogonal receiving antennas. Nine parameters that are calculated by the receiver, in combination with one known position or orientation parameter, determine the location and orientation of the receiving antennas. In another embodiment for measuring two dimensional (planar) position and orientation, the patent describes a system having two orthogonal radiating antennas and two orthogonal receiving antennas. In all the embodiments of Patent '881, the computation is based on an iterative computational scheme.
U.S. Pat. No. 4,287,809 to Egli et al, describes a helmet locator system in an aircraft consisting of a receiving antenna associated with the helmet and two transmitting coils fixed in the cockpit of the aircraft. The receiving antenna comprises three orthogonal field detectors. The position and orientation of the helmet are determined by measuring at the helmet a set of three different magnetic field vectors transmitted by the sequentially activated transmitting coils.
U.S. Pat. No. 4,314,251 to Rabb describes a system comprising either two orthogonal radiating antennas, and three non coplanar receiving antennas, or three radiating antennas and two receiving antennas. The system is based on time multiplexing of the radiating antennas to identify the source of the radiation.
U.S. Pat. No. 4,849,692 to Blood describes a similar system to that of Raab, except that a D.C. magnetic field is used instead of an A.C. field as in Raab.